1. Field of the Art
The present invention relates to a commutation compensation device for a DC machine of the type in which in addition to an interpole coil for armature current flow, an interpole is provided with an auxiliary interpole coil, and in response to a reference exciting current calculated based on a rate-of-change of the armature current the exciting current is caused to flow through a controlled DC power source to the auxiliary interpole coil for compensating commutation.
2. Prior Art
In general, the commutation of a DC machine is satisfactory when there exists a predetermined value of interpole flux in proportion to the armature current, but the commutation is degraded when the auxiliary interpole flux rises above or drops from said predetermined value of interpole flux. When the rise or fall of the value of the interpole flux is compensated by the magnetomotive force, it becomes possible to prevent the degradation of commutation and a device for exciting the auxiliary interpole coil by a special exciting power source so as to prevent the degradation of commutation is a commutation compensation device.
Too high or too low value of interpole flux is caused when the eddy current produced in the magnetic circuit due to rapid change of the armature current causes a time lag in the case of variations of flux. Furthermore, in the case of a DC machine having a great difference between the base speed and the maximum speed, the displacement between a high speed and a low speed of a non-sparking center becomes too long. Therefore, when the value of the auxiliary flux is adjusted for a high speed, the value of interpole flux becomes too low in the case of a low speed and on the other hand when the adjustment is made for a low speed, the value of interpole flux becomes too high in the case of a high speed.
Whether the commutation compensation device can accomplish its function effectively or not is dependent upon a degree of compensation; that is, the determination of a compensation rate. However, there arise various problems as described below in the case of the compensation of the degradation of commutation due to a sudden variation of the armature current. For instance, it is very difficult to estimate the relationship between the delay of flux and the sparking by analysis and also it is difficult to determine the relationship between the delay of the exciting current and the sparking during the tests conducted in a factory. Furthermore, in order to compensate for the displacement due to the speed of the center of the non-sparking band, even when the optimum adjustment has been accomplished in the factory, it varies in response to the variations of the commutation characteristic resulting from the formation of a commutation film after the operation. Thus the determination of the compensation rate is considerably difficult and cumbersome. In the case of the conventional commutation compensation devices, the determination of a compensation rate; that is, the accuracy of the commutation compensation is not satisfactory so that the improvements of the commutation capacity and the overload characteristic of a DC machine are not satisfactory in practice.